If multiple nonblack creatures enter the battlefield at the same time, Grave Peril will destroy only one of them. Grave Peril’s ability will trigger multiple times, but only the first one to resolve will do anything.

5/1/2007

If a nonblack creature enters the battlefield and Grave Peril triggers, then another nonblack creature enters the battlefield, Grave Peril will trigger again and destroy only the last creature to enter the battlefield.